Marine drives can conveniently be classified into three categories.
These are:                a. Inboard motors;        b. Outboard motors;        c. Stern drives.        
Inboard motors and outboard motors are discussed in the preamble of U.S. Pat. No. 6,186,845 which discloses an embodiment of the type of drive known as a stern drive. In this type of drive the motor is mounted on or immediately inboard of the transom of the boat with its drive shaft passing through the transom and downwards within a fairing outside the boat's hull to the gear set and propeller shaft which are at the lower end of the fairing.
A technical complexity which has to be dealt with in a stern drive results from two factors. Firstly, the fairing must be able rotate about a vertical, or substantially vertical, axis so as to direct the propeller's thrust at an angle to the front-to-rear line thereby to permit steering. Secondly, it must be possible to “trim” the fairing, which means tilting the fairing about a horizontal axis to change its pitch. This directs the propeller's thrust either horizontally or at a desired angle with respect to horizontal. This movement is also used for the purpose of raising the fairing so that the boat can be loaded on a trailer or run onto a shore.
U.S. Pat. No. 6,186,845 discloses a stern drive which permits the steering motion of the fairing and also the tilting motion of the fairing which is needed to adjust the fairing's pitch and permit it to be raised to enable the boat to be placed on a trailer.
PCT specification WO 2004/085245 discloses another form of stern drive. Without in any way attempting to provide an exhaustive list, other forms of stern drive are disclosed in U.S. Pat. Nos. 6,468,119, 5,601,464, 4,037,558, 3,847,108 and 3,166,040.
Conventional stern drives are based on layouts in which the crank shaft of the engine drives an output shaft through a universal joint, or more usually two universal joints. Constant velocity joints have been proposed as substitutes for universal joints. The output shaft is horizontal, or substantially horizontal, and drives a gear set, the output shaft of which is vertical or substantially vertical. The vertical output shaft drives a lower gear set which in turn drives the propeller shaft.
A gimbel is provided which carries the motor and which is mounted on a fixed part of the boat. The gimbel is usually mounted for motion about a vertical, or near vertical, axis. A steering arm is connected to the gimbel. By rotating the gimbel about its vertical mounting axis, the gimbel and the entire fairing are displaced about the vertical axis of the gimbel thereby directing the thrust of the propeller at an angle to the front-to-rear line of the boat and enabling it to be steered.
The mounting of the fairing on the gimbel is about a generally horizontal axis. By tilting the fairing about this horizontal axis with respect to the gimbel using one or more rams, the fairing can be trimmed up or down and lifted for stowage.
The universal or constant velocity joints provided between the crank shaft and the horizontal output shaft permit these shafts to move relative to one another as the fairing moves with the gimbel (about a vertical steering axis) and with respect to the gimbel (about a horizontal trim axis).
A modification on this standard system has recently become available commercially. In this form the gimbel is mounted on the boat for movement, with the fairing, about a horizontal axis to enable the fairing to be trimmed. The fairing is mounted on the gimbel for movement with respect to the gimbel about a vertical axis. The steering arm displaces the fairing with respect to the gimbel about this vertical axis for steering purposes.
The mounting structure of U.S. Pat. No. 6,186,845 avoids the use of universal joints but has the disadvantage that the entire motor and fairing moves during trimming motion. This means that a space, in addition to that occupied by the motor in its normal position, must be provided and into which space the motor can move when the fairing is raised for stowage purposes.
The gear set of conventional stern drives as described above, can include a first bevel pinion driven from the crank shaft of the motor, first and second bevel gears meshing with the first bevel pinion and being rotated in opposite directions, a reversing clutch for connecting the first bevel gear or the second bevel gear to a first transverse shaft. The first transverse shaft will thus rotate in opposite directions, depending on whether the first or the second bevel gear are connected to it. The rotation of the first transverse shaft is transferred to the output shaft.
The first and second bevel gears are coaxially carried on the first transverse shaft on opposite sides of the first bevel pinion and the clutch is thus used to connect either the first or the second bevel gear to the first transverse shaft in order to change the rotational direction of the output shaft between a forward and a reverse condition. Each of the first and second bevel gears can have a protruding part that defines a conical clutch face and the clutch can include a clutch element, connected to the first transverse shaft with helical splines, between the first and second bevel gears. The clutch element can be connected to either the first or the second bevel gear, by sliding axially on the first transverse shaft and engaging the conical clutch face of one of the bevel gears.
The helical splines are oriented so that, if the clutch element is connected to one of the first or the second bevel gears and transfers torque from the bevel gear to the first transverse shaft, the clutch element is drawn into engagement with the particular bevel gear by the interaction between the clutch element and the splines. The result is that the clutch keeps itself in engagement, while torque is being transferred and little force is required to engage it. However, the force that is required to overcome the self engaging spline action and thus to disengage the clutch, can be quite high. The mechanism by which the clutch element is shifted on the first transverse shaft thus has to be capable of effecting substantial axial forces on the clutch element.
In gear sets of this kind, the clutch is conventionally operated by sliding the clutch element on the first transverse shaft, with a fork-shaped selector, engaging the clutch element in a circumferential shifting groove. However, selectors of this type, that obviously have to be clear of the bevel gears, require space, which comes at a premium in these gear sets and the spacial requirements of these selectors inhibit the development of compact new types of stern drives. It should be borne in mind that the gearset is aft of the transom and the hydrodynamics of the marine drive can be severely affected by the size of the gear set, the gearbox casing, the cylindrical housing, etc.